Just my personal opinion, take it for what it is worth.
Most engines have a bypass valve inside the oil filter adapter or engine itself. If you oil filter becomes plugged or blocked off for any reason, the bypass valve allows dirty oil to go around that filter and through the engine as normal. Most engineers feel that dirty oil getting through the engine is much better then no oil at all. Kind of a "duh," eh?
Well, Cummins didn't add that bypass on the 6BT. So, the oil MUST travel through the oil filter in order to get throughout the engine. On top of that, the engine moves an insane amount of oil through the filter. If I recall correctly, it is in the neighborhood of 150gph.
So, my personal opinion is that you really shouldn't be taking that high pressure, high volume, non-bypassable oil away from the block, through small oil lines, and back to the block. Amsoil does not recommend their dual filter (1 regular filter and 1 bypass filter) on engines older then 03, if at all. (They have changed their policies a few times).
Also, I would not be too concerned about "black oil." The color of oil has nothing to do with the condition the oil is in. The only way to know what condition the oil is in is to have it sampled through a reputable lab. Most of the "black" in the oil comes from tiny particles of soot. That soot can actually be sub-micronic in size. To give you an idea, a human hair is around 70 micron. Most bacteria and red blood cells are around 8um (8 micron). So you are talking about particles many times smaller then a red blood cell in your oil.
The concern is when those particles of soot band together and build in size. This is where a good bypass filter comes in. Once those soot particles gather to a size that the bypass filter will filter out (measured in absolute size, since most of the time it will pick up some smaller particles, although not all, in what filter manufactors call their "nominal rating.") The absolute micron filtration rating is usually hovering around a 98% efficiency.
A good full flow filter will usually be rated around 20-25um Absolute, 12-15um Nominal. The problem is that some research has shown damaging particles are around 10-12um.
This is where bypass filtration comes in. A good bypass filter is going to be rated around 6-8um Absolute, and around 2-3um Nominal. Some are even capable of going lower then that. Toilet paper elements are not researched by the big labs as much, but some claim sub-micronic ratings both nominal and absolute.
If I recall correctly again, the FS-2500 is rated somewhere around 1-2um absolute. Amsoil is rated around 4-5um absolute. I would really need to go back and read their publications to know for sure, but I'm thinking it is somewhere around there.
You can also build your own bypass filters. Hastings/Baulding filters have a bypass filter (the B50) that is rated around 2-3nominal, 8um absolute. Until someone releases a peer-reviewed, hard studied paper showing that we need to filter oil down lower then 8um (or 10-12um for that matter) in a diesel engine, I'm not paying the extra cost for those big company bypass filters, and I'm not dealing with the mess of a toilet paper element bypass filter.
Just my 2 cents.
